(1) Field of the Invention
The present invention relates to semiconductor devices applicable to power switching elements, for example, in power supply circuits of consumer appliances such as a television.
(2) Description of the Related Art
In recent years, field effect transistors (FETs) made from a gallium nitride (GaN)-based material have been intensively studied as power switching devices. A nitride semiconductor material such as GaN forms various types of mixed crystal together with aluminum nitride (AlN) and indium nitride (InN) and therefore forms a heterojunction like the conventionally-used arsenic-based semiconductor material such as gallium arsenide (GaAs). In particular, a heterojunction of a nitride semiconductor has a characteristic that spontaneous polarization and piezoelectric polarization produce a high concentration of carriers at the hetero-interface even when the nitride semiconductor is not doped. As a result, FETs using nitride semiconductors are likely to exhibit the depletion mode (normally on mode), and it is thus difficult to obtain characteristics of the enhancement mode (normally off mode). However, most of the devices used in the power electronics market today are in the normally off mode, and there is therefore a strong demand for GaN-based nitride semiconductor devices in the normally off mode.
As a structure which provides the normally off FETs, a junction field effect transistor (JFET) has been proposed in which a P-type GaN layer is formed in the gate region (see Patent Reference 1: Japanese Patent Application Publication NO. 2005-244072, for example). In the JFET structure, piezoelectric polarization occurring at the hetero-interface between the undoped GaN channel layer and the AlGaN barrier layer is offset by piezoelectric polarization occurring at the hetero-interface between the AlGaN barrier layer and the P-type GaN layer. This makes it possible to lower the concentration of two-dimensional electron gas in the P-type GaN layer right below the gate region, thereby providing a normally off characteristic. In addition, when the gate includes a p-n junction having a larger built-in potential than a schottky junction, the gate turn on voltage can be higher, which produces an advantage that the gate leakage current can be smaller even when a positive gate voltage is applied.